csed used car prices to sore. with new car sales down, there are fewe late model trade-ins and that affects supply and demand. dealers are keeping older vehicles on their lots, rather than sending them to auction. when drivers keep cars longer rather than trading them in, dealers must pay top dollar for used car inventory. speaking of keeping your used car running smthly, how far should you drive before changing the oil in your car? break that habit of every 3,000 miles. it's a myth and ss because of technological advances, an average car's interest val is aroundp 7,800 miles now. the porsche can go 20,000 miles before an oil change. service centers and deers use the 3,000 mile change as a marketing tactic to get you into the service bay on a regular basis. the quick oil change industry defends that 3,000 mile rule. because busy car owners seldom read the driver's manual, they have no idea how often the oil should be changed in their car. >>> phonily, are you tired dropped calls on your cell phone? jd powers and a